So people know that MOST starfish have five arms. But a bunch of starfish species all over the world have 6 ore even more arms..many have about 10-15 arms..up to 50 in the Antarctic Labidiaster.

Among the better known of these is the predatory Sunflower star (Pycnopodia helianthoides) from the west coat of North America....
Multi-armed starfish occur in tropical (for example, Coronaster) and cold-water (for example, Labidiaster or Solaster) environments as well as in shallow-water (Coscinasterias or Pycnopodia) and deep-water habitats (for example, the Pacific California Rathbunaster or the brisingids).

The legendary Manx naturalist Edwin Forbes described the fossil in 1850 (this is the Forbes for whom the Atlantic starfish Asterias forbesi is named), calling it Lepidaster grayi.

The name is composed of the Ancient Greek for "scale"...Lepidos and -aster for "star" for the scale-like body elements. Forbes named the species for John Gray, one of Forbes' colleagues and fellow scientists.

Based on Liam's accounts, it apparently not clear to Forbes if his fossil was even a starfish!

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As author-illustrator of the definitive British guide, Forbes knew his living starfish, but this fossil baffled him. At first it reminded him of the common sun star Crossaster papposus, individuals of which often have 13 arms, but Forbes began to have his doubts. Was this palæontological oddball even a starfish at all? Could it be the missing link between starfish and sea lilies, their many-armed cousins?

Liam's paper breaks down the evolutionary importance of Lepidaster and what it means to be a PALEOZOIC multi-armed starfish! Let's go through the major points!

1. This is the earliest fossil where we find multi- armed starfish in the Fossil Record!
If we were to compare this to vertebrates, this might be like looking at all of the mammals, turtles, lizards, snakes, dinosaurs, crocodiles, etc. and trying to find the very FIRST time that something stood up and walked around regularly on two legs. A minority of mammals do it today-but wouldn't it be interesting if some OTHER species did it before??

So, multiple arms is a recurring evolutionary theme in sea stars. Most have five-but it occurs over and over across very different evolutionary lineages. More on this below under #2...

This tree basically shows APPROXIMATELY where Lepidaster is placed relative to modern starfishes and how far away from the starfishes you see in the oceans today would be from something fossil like Lepidaster.

2. Multiple Arms appear Multiple Times in the Evolution of Starfishes!

So, not only did multiple arms appear first, but it is the first of MANY times that we see multiple arms throughout starfish evolution.

A tree of starfish is below...the "starbursts" on the right give you an idea of all the different evolutionary branches where the multi-armed condition is seen..

...So bottom line is that having multiple arms is the result of what's called CONVERGENT evolution also known as homoplasy.
That is to say that the SAME body form or structure is seen in two or more completely separate evolutionary lineages. For example, dolphins and whales, superficially resemble fish-with fins and the teardrop body shape, because they live in open ocean/water, etc.... But aside from both being vertebrates, they are NOT directly related.

The same is often observed in starfish! Having multiple arms works in starfish evolution. A hit that keeps on coming back!

3. The Ecology. Why have multiple arms??Was Lepidaster the first??

Liam has provided some intriguing speculation on the nature of the multi-armed condition...

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Does having extra arms help?

If the ratio of body size to arm length is kept consistent, but eight extra arms are added, the volume (or biomass) of a multiradiate starfish is roughly double that of a 'normal' form. This means a marked increase in the energy required to keep the animal functioning. If you need more food, there are two ways you can go about getting it. You can out-muscle your competitors for existing food sources, or you can start exploiting new ones. Many starfish with supernumerary rays seem to have taken the latter course of action, from consuming coral,
to eating other echinoderms, even to scoffing (=eat) fellow starfish. Perhaps Gray’s scaly star was the pioneer. With its mouth in the middle of a much larger and more flexible body than that of its contemporaries, it would certainly have been capable of doing something different.Herringshaw and his colleagues speculated that Lepidaster might be similar to the solasterid Crossaster. Here for comparison against the mouth shown above is a related solasterid, Heterozonias, (a deep-sea predator) which looks similar...

To further explore Liam's notion's we often invoke a principle known as uniformitarianism which basically assumes that the "present is the key to the past". In other words, processes that are at work today are those SAME processes that occurred in the past.
And so, we look to modern starfishes for possible ecology, feeding strategy, and evolutionary ideas about how the multi-armed condition arose.

Specifically, we look at the functional morphology of modern forms, in other words, any kind of direct behavioral or ecological inference based on the external morphology-like how sharp teeth are used to tear and eat flesh.....something that we hope will surrender clues or provide inference of what the fossil animal's life mode might have been like...

A good example of different multi-armed species that have a specific ecological function associated with their form are taxa like the Brisingids, who use their arms to pluck food out of the water as suspension feeders. I wrote these up here.If we follow up with an example of what a modern predator like Solaster looks like...
Compared to Lepidaster...
There's a LOT of differences in the fundamental skeletal structure between these two animals. Bear in mind that ANY similarlities are almost certainly because of convergence! (Solaster is a highly derived modern starfish)

That being said, the overall form of the body between Lepidaster and solasterids, such as this predatory Arctic sun star Solaster endeca is curiously similar.
This sort of implies thatLepidasterprobably was some kind of predator. But the similar body form COULD be due to some entirely OTHER evolutionary adaptation!

Thus, the mystery may remain unresolved, barring better fossils preserving catching Lepidaster eating prey (or the unlikely discovery of a living Lepidaster somewhere), BUT we have a much better idea of what this mysterious and intriguing beast was like and considerations about its lifestyle and evolutionary "story".....
My, grandma starfish, what a lot of arms you have!

All the better for grabbing you with!Why grandma starfish, what a flexible mouth you have!

Tuesday, April 20, 2010

Today's topic: STARFISH Defense!!Its curious how often this question comes up. People see starfish and other echinoderms that are just sitting there. Echinoderms don't work in our time scale-and to our perception, they just kind of sit there.

But starfish? How are these seemingly innocuous creatures able to protect themselves?How do they get away? When things come after them? What comes after them?Today...a survey of what sea stars use to defend themselves against their various predators and adversaries.

What are starfish usually worried about? Usually fish...but also crabs, and sometimes other echinoderms...

1. Chemical Defenses. So, this is the one most people seem to miss, but its probably one of the most important. Most starfish have specific type of organic chemical called SAPONINS in their body wall.Among animals (saponins are also found in plants), saponins are only present in sea cucumbers and starfish. Saponins are very unpleasant tasting and based on studies, such as this one and this one, are found in the larvae as well as the adults.

So, fishes and other predators basically learn that starfish are just AWFUL. In vertebrates, such as cats and/or dogs eating starfish and being poisoned by sapoinins can cause vomiting, nausea, and other unpleasant effects. Humans should take this hint and go with it...

Some species, such as this Astropecten polyacanthus have kicked their defenses up a notch and instead of (or possibly in addition to?) saponins, their body walls contain the deadly TETRODOTOXIN.

(photograph by Cory Pittman)

Tetrodotoxins have this chemical structure and function as nerve-blockers that can result in a variety of unpleasant outocmes including death. Pure Tetrodotoxin is 100 times MORE poisonous then potassium cyanide.Tetrodotoxin gets its name because it is naturally occuring in puffer fish (order Tetrodontiformes). Wikipedia has an interesting article on it here.

Chemical defense also include the production of SERIOUS amounts of toxic materials such as MUCUS (grammar factoid! Mucus=noun. Mucous=adjective!) in the "slime star" Pteraster tessellatus (and probably all of its relatives in the Pterasteridae.)This picture is no exaggeration. I've seen these animals produce BUCKETS of mucus in response to being annoyed. Early studies of Pteraster in the lab showed that other animals clogged by the mucus eventually die, suggesting that its not a substance that likely predators are fond of....

2. Armor & Spines. One of the more obvious features one sees in starfish, especially in tropical species is the presence of heavily developed armor and armament, such as the spines in this Protoreaster nodosus.To be honest, armor and spines as defensive are usually interpretations of how the body forms function in the wild. Tropical starfish in the family Oreasteridae, such as Pentaster and Pentaceraster (below) almost all have these heavily developed bodies.One major group of starfishes, the Valvataceans, which includes the Oreasteridae and other starfish groups with heavily calcified bodies, are highly diverse and considered evolutionarily successful in tropical regions, especially in the tropical Indo-Pacific.

For example, here's the underside of the starfish Tosia. The tube foot groove is flanked by thick blunt spines.These spines close up over the tube foot groove and clasp together. This is thought to protect the animal's tube feet and soft tissue from small predators, such as little shrimps, worms and fish.

In the New Zealand Pentagonaster, there are big, thick armor plates that cover over the armtips.These are thought to similarly protect the armtips from fish, crabs, and/or other predators that attack the armtips.

If predation pressure is that much higher in the tropics-that may drive the evolution and diversification of tropical starfish species..

The crown of thorns starfish, Acanthaster planci is a big ol' starfish bad-ass. Its got the morphological defenses...the sharp spines AND it has saponins in its body wall.Even if you look at something familiar like the intertidal Pisaster live in harsh wave-swept regions and have heavily calcified armor that I would say protects them the elements and other predators.

They hypothesize that larger individuals have "increased mechancial toughness" and that this replaces the shedding of arms (i.e., autotomy) as an antipredator strategy. There may be a size-related decline in the efficiency of the autotomy mechanism through the relaxation of selection pressure (i.e., the influence of predators).This was an idea applied to individuals within this species..but if true, then large size itself could be another defense!! One possible explanation for why oreasterids are so big?? Or possibly other species?

Tuesday, April 13, 2010

So, during a tour I gave the other day, I was explaining just how it was that something as offbeat as studying starfish evolution IS actually pretty similar to anthropology and human forensics.

Or, to put it more simply...what I and some of my colleagues do is really NOT that much different then what they do on the TV show "Bones"...The premise of the show is that the namesake "Bones" (=Dr. Temperance Brennan) can reconstruct a history of what happened to an individual by looking at and analyzing their bone structure and INFER the history (and thus, the criminal events) of what happened to that individual.Believe it or not, a starfish (or even a brittle star) is NOT all that much different. Let's take for example, this goniasterid starfish, Ceramaster.Asteroids (and other echinoderms) are made up hundreds of thousands to millions of small calcium carbonate bits infused with tissue that are made into a living 3-dimensional box aka the BODY of the starfish.Kind of like those 3-D puzzles that you see around...These individual pieces, called OSSICLES or sometimes more informally-PLATES compose the body of not only starfish but all echinoderms. It is the arrangement of these plates that allow us to identify and classify them.Ossicle architecture and arrangement are analogous to the way bones are understood in humans (and other vertebrates). And by knowing where these "bones" (=ossicles) are located, we can use them to reconstruct an animal...and (within limits) its history.This comes in quite handy for paleontologists who study fossil starfish!

Fossil starfish remains are often broken apart and disarticulated (i.e., they are discovered as individually separated pieces). Trying to put them back together to resemble the formerly living animal can be quite a challenge.

There is a HUGE literature of European starfish paleontology that focuses exclusively on individual ossicles like this. One great website with images of individual ossicles and animals can be found here.

In some places, you can literally find bucketloads of individual ossicles! Sometimes they are so abundant, they can actually be used to identify stratigraphic rock layers. Very RARELY are starfish fossils ever found intact.

Okay...so ossicles are nice...but how is that ANYTHING like being the starfish version of Temperance "Bones" Brennan ???

What can you tell about a starfish as it lived from dead pieces?If we observe how the SAME series of ossicles in parallel position on the animals' body, we can see how those ossicles series change across different species. In vertebrates we would look at how those same BONES have changed across different animals

In other words, how have those bones/limbs have EVOLVED or ADAPTED to different environments.We can do the same thing with starfish!!What you're seeing below is the above Ceramaster with its surface body wall completely removed.

That's the mouth at center with each of its five arms projecting from what's called the mouth frame.

Each arm series (kind of like an arm or a leg in a vertebrate) is made up of a very distinctive series of ossicles called the ambulacrals. The ambulacrals are often specific to different kinds of starfish species!Let's take this Pacific Asterias amurensis for example...If you take the top off..you see that there's a similar but different structure.

The individual ambulacral plate/ossicle series are made of of much thinner, smaller pieces as you can see here...These suggest that there is much more flexibility and range of movement in the arm, which is what you would expect for Asterias, which feeds on mussels and other shellfish versus Ceramaster which is more of a less active, sit-on-the bottom type deposit/opportunistic bottom feeder.In contrast, we look at a very opposite feeding type...the brisingid, filter feeder Novodinia antillensis.I've written up how these feed here...

But long story short, they hold their arms up into the water column to pluck tasty crustaceans and other tiny food out of the current as it flows by....The ambulacral ossicles on Novodinia look like the vertebrae from a backbone!! These permit the arms to be very long and stable but ALSO to FLEX! and curve into the water.

And finally...just to show you another extreme, here's a pic of Culcita novaeguineae, a living tropical Indo-Pacific "cushion star"

And here is what happens when you cut one open!

Much of the inside is filled with water (i.e., empty) but you've got the massive armored body wall and you actually need a "support pillar" inside the body cavity that connects between the body wall and the ambulacrals.

The mouth frame is pretty thick and robust...and the ambulacrals actually have kind of a special cavity for the tube feet to pull up into when the animal doesn't want any of its soft parts exposed!

The weird insides of this cushion star are actually pretty typical for ALL oreasterids.

So, you could infer from the individual ossicles or even a badly damaged specimen that you've NEVER seen alive before, the possible KIND of life mode it had! Not always...but it is one way to do the job!

About Me

I pursue starfish related adventure around the world with a critical eye and an appreciation for weirdness.
Support has been courtesy of the National Science Foundation but the views and opinions presented herein are mine and do not reflect the opinions of them or any affiliated institutions.
Need to hire an invertebrate zoologist/marine biologist? Please contact me!